Move AbortController/AbortSignal to isolated lib file to share definitions between DOM and NodeJS

[rbuckton]

NodeJS introduced a global AbortController/AbortSignal in Node 15.x. It would be helpful if NodeJS could share the definitions from the DOM using /// <reference lib="abortcontroller" /> comment.

This was mentioned in https://github.com/DefinitelyTyped/DefinitelyTyped/pull/52357/files#diff-2696281f8832b1227bb41483a7525a8ccd9399bb4b1704f69c8c54bb1346d8edR315

Related: DefinitelyTyped/DefinitelyTyped#52357

[ankon]

For future googlers: AbortController already exists in 14.17 as "experimental", and seems to be needed to consume @types/node 14.17.28:

> tsc --outDir ${BUILD_DIR:-build}/plugins -p tsconfig.plugins.json

node_modules/@types/node/child_process.d.ts:267:18 - error TS2304: Cannot find name 'AbortSignal'.

267         signal?: AbortSignal | undefined;
                     ~~~~~~~~~~~


Found 1 error.

Adding a reference to the "dom" lib in tsconfig fixes that, but feels "wrong". Alternatively moving to ^14.17.0 <14.17.28 for the version of @types/node helps as well.